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8/05/2013

Supernova and Kilonova.

Lifecycle of the Sun

I love supernova, whether they are Type I or Type II. I find it fascinating that a star greater than 8 times the mass of the Sun will live a very short life (in astronomical terms) and end their life by blowing up, leaving either a neutron star or a stellar black hole. A star like our Sun, with a binary companion, after going through its red giant stage, will shed its outer layers and then the core will become a hot object, called a white dwarf, about the size of the earth. This white dwarf will cool over time. If it is like our Sun, it will be a planetary nebula for a thousand or so years until its outer layers drift off, no longer illuminated by hot white dwarf leaving only the white dwarf. The white dwarf is no longer having thermonuclear reactions, all reactions have stopped. The white dwarf is not generating heat, it it simply hot, usually having a carbon oxygen core that is very hot and takes billions of years to cool. Now if that white dwarf is a companion to another star, is close enough in orbit to steal mass from its companion, as that companion star nears the end of its life it will swell into a red giant star and the white dwarf, if it is close enough, will begin to pull mass off the red giant. You can see this in this image and read about the process in depth from this link.

To provide an example of how big a star like our Sun is when it becomes a red giant is best seen in this image.

Now as the white dwarf continues to steal mass from the red giant companion star, the mass of the white dwarf begins to increase. When the white dwarf accretes and reaches a limit known as the Chandrasekhar Mass limit (1.44 solar masses which means 1.44 times the mass of the current Sun; though this is listed as 1.4 solar masses in some sources) the white dwarf begins to undergo nuclear fusion and in a matter of seconds, the remaining mass, carbon/oxygen or if the original star is larger than our Sun, the white dwarf will be comprised of a oxygen-neon–magnesium core, collapses and the white dwarf explodes into a Type I Supernova. This link shows on PBS NOVA how this process works.

Just to clarify, in some cases both stars in a binary system will become white dwarfs without the first one exploding as a Type I Supernova. In this case the gravity of the two white dwarfs can be drawn to each other in a dance of death, until when they merge, their mass exceeds that 1.44 solar mass limit and they explode as a Type One Supernova.

When the white dwarf explodes, the explosion is massive. If it had a companion star, that star has some of its outer atmosphere blown off and the remaining star is sent hurtling faster than the other stars in the area of space around it, fleeing from the explosion. There is nothing left of the white dwarf that explodes. Unlike a Type II Supernova where a massive star explodes, leaving a neutron star/pulsar or a stellar black hole. These massive stars that cause a Type II Supernova, are so large, some have an orbit out between Mars and Jupiter

(as shown in this picture). This image shows how big Betelgeuse is compared to our Sun, the red dot in the middle and the star Deneb. Both Betelgeuse and Deneb are large enough stars that they will end their lives as supernova.

Betelgeause from above has created and burned through most of these layers. A massive star will burn through these layers in this amount of time per layer:

Star burns through a succession of nuclear fusion fuels:

Hydrogen burning: 10 Myr

Helium burning: 1 Myr

Carbon burning: 1000 years

Neon burning: ~10 years

Oxygen burning: ~1 year

Silicon burning: ~1 day

Then it forms Iron, and in milliseconds of the core exceeding 1.4 masses of Iron, BOOM, the Supernova explodes. A star like our Sun will burn for about 10 to 12 billion years and the Sun per above, has been burning for just over 5 billion years.

As massive giant stars age, they produce "onion layers" (see above) of heavier and heavier elements in their interiors. However, stars will not fuse elements heavier than iron. Fusing iron doesn't release energy. It uses up energy. Thus a core of iron builds up in the centers of massive supergiants.

Eventually, the iron core reaches something called the Chandrasekhar Mass , which is about 1.4 times the mass of the Sun. When something is this massive, not even electron degeneracy pressure can hold it up.

At this point, the core collapses and two important things happen:

1. Protons and electrons are pushed together to form neutrons and neutrinos in the core.
2. Even though neutrinos don't interact easily with matter, at densities as high as they are here, they exert a tremendous outward pressure.

The outer layers fall inward when the iron core collapses. When the core stops collapsing (this happens when the neutrons start getting packed too tightly -- neutron degeneracy), the outer layers crash into the core and rebound, sending shock waves outward.
These two effects -- neutrino outburst and rebound shock wave -- cause the entire star outside the core to be blow apart in a huge explosion: a type II supernova!
Supernovae are really bright -- about 10 billion times as luminous as the Sun. Supernovae rival entire galaxies in brightness for weeks. They tend to fade over months or years.

During the supernova, a tremendous amount of energy is released. Some of that energy is used to fuse elements even heavier than iron! This is where such heavy elements like zinc and uranium come from!

The material that gets ejected into space as a result of the supernova becomes part of the interstellar medium. New stars and planets form from this interstellar medium. Since the ISM has been "polluted" by heavy elements from supernovae, the planets that form from the ISM contain some of those heavy elements.

The collapsed core is also left behind by a type II supernova explosion. If the mass of the core is less than 2 or 3 solar masses, it becomes a neutron star. If more than 2 or 3 solar masses remains, not even neutron degeneracy pressure can hold the object up, and it collapses into a black hole.

Now this is a massive explosion in either case, a Type Ia or a Type IIa. 10 billion times as luminous as the Sun means these Supernova outshine initially, for several months, the brightness of the galaxy they explode in. If a star is really massive, like this star, VV Cephi:

when they go supernova they often have a burst of energy, released from their two poles, called a gamma ray burst. For massive stars like VV Cephi, these gamma ray bursts are long. However there are short gamma ray bursts that occur and astronomers think they know why.

For stars that leave a neutron star, when a neutron star, which is VERY dense merges with another neutron star or with a black hole, that explosion that occurs Astronomers have long suspected that an item called a Kilonova happens. This is when a neutron star, left from a massive star after the star has gone through the supernova stage, falls into a black hole or merges with another neutron star. As shown in this NASA image:

Credit: NASA, ESA, and A. Field (STScI)

In 1 you have two neturon stars orbiting and falling into each other. In 2 they merge cause a super heated explosion with a lot of radiation. This explosion results in 3. a short gamma ray burst until the new object, probably a black hole begins to devour the remains of the two neutron stars while heating up the remaining gas that surrounds the black hole. Eventually, the stellar black hole will not have the material around it as it devours it or spits it back out into space and it will remain unseen at that point. That is what a Kilonova is. Hard to observe as they are very short. If you want to learn more and have a better picture in your head here are some links:

Space.com and they have a good video of the images put together showing the Kilonova.

3 comments:

love the brief synopsis. i've noticed that in one of your comments you state that in a type II supernova that if the iron core reaches the 1.44 solar mass limit it will trigger a supernova. then you state that if the star has < 2 or 3 solar masses remaining it will form a nuetron star and if > 2 or 3 it will form a black hole. i find that in agreement with most web sites. but i often have wondered. if the core always collapses at 1.44 solar masses shouldn't that be all that remains? how is it that it gains over 1.44 solar masses? or perhaps there is some elementary part of the equation that i am overlooking?

To clarify, in the writing I believe I point out that when the iron core reaches 1.4 solar masses, it causes a collapse that triggers the supernova. The mass and changes that take place regarding the development of neutrinos, protons and electrons as well as other chemical changes cause the core to evolve into either a neutron star or a black hole. The remaining mass of the star is blown outward in the shock wave. Thus there is the remains of the star in terms of the black hole or the neutron star that forms from the iron core.

Perhaps the best I can offer is from Dr. Richard Pogge's Astronomy 162 notes located here: